Transformer heating in current probes has always been a problem with respect to the customer""s safety and the probe""s reliability. Measuring higher currents at higher frequencies continues to push the envelope on what probes/amplifiers can provide. In the past, derating curves have been used to indicate the safe operating areas of the probe but could not detect situations in which a customer was exceeding the derating curves. The ability to constantly measure and monitor the temperature of the transformer allows control of the maximum temperatures that the probe can reach and limit potential damage to the probe as well as safety issues for customers. The below described invention disclosures a method for monitoring the transformer temperature in current probes.
Accordingly, the present invention is a method of monitoring the temperature of a transformer winding in a current probe. The current probe includes a magnetic core having a multi-turn winding disposed there-around forming a probe transformer. A Hall Effect device is disposed within the magnetic core for generating a differential output signal for producing a current signal through the multi-turn winding. The current probe transformer temperature monitoring method determines an initial transformer temperature of the current probe as a function of the winding resistance of the transformer. A relative temperature of the Hall Effect device is also determined as a function of resistance change of the Hall Effect device. The initial transformer temperature and the relative Hall Effect device temperature are combined to produce a continuous transformer temperature indicative of the temperature of the transformer. The current signal to the multi-turn winding is removed when the continuous transformer temperature exceeds a threshold temperature value and a visual indication may be provided.
The initial transformer temperature determining step includes the steps of storing the thermal coefficient of copper, xcex1, an initial transformer temperature, T0, and a termination resistance, Rtermination, in memory. Digital values representative of an input voltage, Vin, to the multi-turn winding and an output voltage, Vout, from the multi-turn winding are generated and an initial probe resistance, R0, is calculated using the termination resistance and the digital values of the input and output voltages. The initial transformer temperature, Tprobe, is calculated using the function:       T    probe    =            T      0        +                  1        α            ⁢              (                                                            R                termination                            ⁡                              (                                  Vin                  -                  Vout                                )                                      -                                          R                0                            ⁢              Vout                                                          R              0                        ⁢            Vout                          )            
The relative temperature of the Hall Effect device determining step includes the steps of storing a thermal coefficient of resistance value of the Hall Effect device, xcex1H, a Hall Effect device bias voltage source value, VBias+, and a resistance bias value, RBias, in memory. A digital value representative of a voltage, VHall+, across the Hall Effect device is generated and an initial Hall Effect device resistance value, RHall, calculated using the function:       R    Hall    =      (                  2        xc3x97                  V                      Hall            +                          ⁢                  R          Bias                                      V                      Bias            +                          -                  V                      Hall            +                                )  
and stored in memory as RHall Init.. Additional digital values representative of the voltage, VHall+ are generated and Hall Effect resistance values, RHall, representing changes in the resistance of the Hall Effect device as a function of temperature are calculated. Changes in temperature of the Hall Effect device, xcex94TprobeHall, are calculated using the function:       Δ    ⁢          xe2x80x83        ⁢          T      probeHall        =            1              α        H              ⁢          (                                    R            Hall                    -                      R                          Hall              ⁢                              xe2x80x83                            ⁢                              Init                .                                                              R                      Hall            ⁢                          xe2x80x83                        ⁢                          Init              .                                          )      
temperature value.
Alternately, the first order equation may be modified to include first and second order terms, k1 and k2, to increase the accuracy of the change in temperature calculation.
xcex94TprobeHall=k1(RHallxe2x88x92RHall Init.)+k2(RHallxe2x88x92RHall Init.)2
The objects, advantages and novel features of the present invention are apparent from the following detailed description when read in conjunction with appended claims and attached drawings.